A couple of years ago we had a yarn tasting of EcoButterfly Certified Organic Cottons. I just reordered some of these because I love them a lot.
Why bother with Organic Cotton? Normal cotton has been grown with a LOT of pesticides that are both toxic and persistent as well as synthetic fertilizers that hard on the environment. These cottons have had no processing with harsh chemicals that normally is involved in the production of cotton products, and unmercerized cotton is easier on arthritic joints while knitting. This makes them great for knitting for babies - I personally don't love the chemicals next to baby skin or mine for that matter (I have a lot of washcloths out of this stuff). And hey, they come in a bunch of natural colors - except for the obviously dyed ones there are many shades of cream, browns and greens that occur naturally and even darken with washing.
Eco Butterfly cottons are dyed with vegan friendly vegetable dyes (except for pinks & reds which are cochineal). The company even buys carbon offsets to make up for the transportation of the yarn to the U.S.
We have worsted, and a sport weight cotton and a lace weight Qoperfina which has a fine bit of copper running through it.
And the other yarn in lately is Adriafil Kimura.....a fun DK weight seal striping yarn from Italy.